FIG. 12(a) shows a schematic block circuit diagram of a conventional portable cassette tape player as an example of the BTL output circuit. 1 depicts a read head, 2 a signal reproduction processing circuit including a head amplifier and an equalizer circuit, etc., 3 an output stage amplifier of positive phase side (non-inverted output side), 4 a loud speaker as a load and 5 an output stage amplifier of inverted phase side (inverted output side).
During reproduction, the audio signal recorded on a tape (not shown) is read out therefrom through the read head 1 as a read signal A which is an input audio signal. The read signal A is input to the signal reproduction processing circuit 2 in which a high frequency bias component thereof during recording is removed and, after equalization processing, output as an audio signal B. The thus reproduced audio signal B is finally amplified by the output stage amplifiers 3 and 5 in which output signals C and C' are produced and the loud speaker 4 is driven by these outputs. As a result, a reproduced sound is generated by the loud speaker 4.
It is usual that the transistor amplifiers 3 and 5 include in their input stages input stage amplifiers 3a and 5a for producing a pair of signals. The audio signal B is amplified by the input stage amplifier 3a and made a pair of signals whose phases are different from each other by 180.degree.. These signals are amplified by push-pull transistors Q1 and Q2 which constitute an output stage amplifier to power amplify as an output signal C. The audio signal B is inverted and amplified by the input stage amplifier 5a, amplified similarly by push-pull transistors Q3 and Q4 and power-amplified as the output signal C'.
Describing the power amplification in the output stage amplifier 3 as an example, a voltage of a power line Vcc which feeds the output stage amplifier 3 in accordance to the input signal B is lowered by the transistor Q1 to a voltage level of the output signal C. In other words, the output signal C is produced as a result that an amount of voltage drop by an internal impedance of the transistor Q1 is changed according to a waveform of the audio signal B. In this case, the transistor Q1 handles a voltage difference between the line voltage Vcc and the voltage of the output signal C. As a result, the transistor Q1 consumes a power corresponding to the voltage difference.
Although the output stage amplifier is shown in the Figure as comprising a simple circuit including the output transistors Q1 and Q2, a peripheral circuit including a drive circuit, etc., may be included in a practical circuit construction. The above matter is the same for the output stage amplifier 5. Particularly, in the case of the BTL circuit, although the input stage amplifiers 3a and 5a are constructed by differential amplifiers and there are provided negative feedback from output terminals of the amplifiers 3 and 5 to the inverted input sides of the input differential amplifiers 3a and 5a to which a reference voltage (corresponding to Vcc/2 where Vcc is a power source voltage) is supplied, such is omitted in the Figure since it has no direct relation to the present invention.
Describing the operation of the BTL output stage amplifiers 3 and 5 in detail, when a voltage value of the audio signal B is higher than the reference voltage (Vcc/2), the transistor Q1 on the side of the power source is made active by the output of the input stage amplifier 3a and the transistor Q2 on the ground side is cut off. Further, the transistor Q3 on the side of the power source is cut off and the transistor Q4 on the ground side is made active by the output of the input stage amplifier 5a. And, current corresponding to the voltage value of the audio signal B flows from the power source line Vcc through the transistor Q1, the loud speaker 4 and the transistor Q4 to the ground.
When the voltage value of the audio signal B is lower than the reference voltage, the ON and OFF operations of the transistors are reversed to those mentioned above and current corresponding to the voltage value of the audio signal B flows from the power source line Vcc through the transistor Q3, the loud speaker 4 and the transistor Q2 to the ground.
when the voltage value of the audio signal B is equal to the reference voltage, the respective transistors are in OFF state. In this case, due to the negative feedback to the input stage amplifiers 3a and 5a, output terminals of the amplifiers 3 and 5 become Vcc/2, respectively.
Power consumed by the respective transistors Q1, Q2, Q3 and Q4 when the pair of output stage amplifiers 3 and 5 which operate in reverse phase to each other are provided and are operated in BTL operation in this manner is shown by hatching in FIG. 12(b). In this Figure, power consumption of these transistors are shown by areas hatched with lines in different directions.
Power caused by the voltage drop of the output transistors shown by hatching is dissipated by the power amplifier transistor as heat. Therefore, transistors whose power loss is large are required. Since a large power is consumed there, power efficiency in a case of production of the output signals C and C' by the BTL circuit is low.
This fact is a problem in a portable audio device which is operated by a battery of limited capacity since an operating time thereof depends upon utilization efficiency of power. Further, for such device, it is very important, as a commercial product, that it is operable for a long period of time. Therefore, power consumption of the device should be as small as possible.
U.S. Ser. Nos. 08/199,890 and 08/203,307 disclose such techniques as mentioned above.